Surveillance video from a Best Buy store shows the altercation between Librado Cena and William O’Brien on April 16, 2013. The second clip shows Cena interrogated by Fairfax City Police Detective Michael Boone on April 18, 2013. (Fairfax City Police)

Librado Cena told jurors Monday that he never intended to fight with the driver who had “incessantly” honked at him on the Fairfax City roads last April.

Cena said he wanted to tell the driver off, so he approached William O’Brien in a mall parking lot. Cena, 57, said O’Brien, 63, started a physical altercation and that he punched O’Brien just once in the face in self defense. Cena said he was shocked when investigators later arrested him and told him O’Brien was likely going to die.

“I didn’t believe it at first,” Cena said in the packed courtroom. “I thought the detective was trying to get me to admit to something. I didn’t hit him that hard.”

O’Brien did die.

Prosecutors and police in Fairfax County Circuit Court presented a different version of events, describing Cena as the aggressor.

“Mr. Cena followed the victim into the parking lot,” Fairfax City Police Detective Michael Boone testified. “He parked two rows away from him. He sprinted at him. He came right up on Mr. O’Brien and engaged him.”

Police believe Cena started the scuffle by striking O’Brien on the back of the head. A medical examiner testified Monday that she ruled the death a homicide and said O’Brien died from complications from blunt force trauma to the head.

William Houda, a medical expert, testifying for the defense, said O’Brien would likely not have died from the blow that he received, but he was on a blood thinner called Pradaxa. He said O’Brien’s brain was bleeding and the drug kept the blood from clotting, causing pressure to build dangerously in his cranium.

Cena was originally charged with aggravated malicious wounding, a felony that carries a maximum sentence of life in prison. But prosecutors later dropped that charge, saying it didn’t fit the circumstances of the case. A grand jury indicted Cena for assault and battery. The misdemeanor charge has angered O’Brien’s family, who say it is too light.

Cena said Monday that he was sorry about O’Brien’s death and wished he had kept driving that day.

“None of this would have happened,” Cena said. “He would still be alive. I wouldn’t be here.”

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